Most HVDC transmission systems in use today are point-to-point transmission systems, where electric power is transmitted from one AC system to another. This is an efficient way of transmitting electrical power to/from remote areas, across water, between two unsynchronized AC grids, etc. In many circumstances, however, multi-point HVDC transmission systems, where power can be transmitted to/from at least three different points in one or several AC networks, are desired. A multi-point HVDC transmission system will here be referred to as an HVDC grid, and can also be referred to as a multi-terminal transmission system. One example of when an HVDC grid can be useful is when connecting (multiple) off-shore wind farms to (multiple) on-shore power grids. Another example is when transferring large amounts of power over long distances in existing AC grids, in which case low loss transmission can be achieved by using an HVDC grid as a backbone or over-lay grid to the existing AC grids.
A drawback of DC transmission as compared to AC transmission is that the interruption of a fault current is more difficult. A fault current in an AC system inherently exhibits frequent zero crossings, which facilitate for current interruption. In a DC system, no inherent zero crossings occur. In order to break a DC current, a zero crossing of the DC current generally has to be forced upon the system.
Moreover, a fault current can grow very rapidly to high levels in an HVDC grid. A fast breaking of a fault current is therefore desired.
Thus, in order to limit the effects of a line fault, a DC breaker should react very fast, typically in the transient stage while the fault current still is increasing and before the DC voltages have collapsed too much. Efforts have been put into the development of fast and reliable DC breakers, and the DC breakers that currently provide the fastest interruption of current are based on semi-conducting technology. A semi-conductor DC breaker is for example disclosed in EP0867998. However, semi-conductor DC breakers designed to break large currents are considerably more expensive than mechanical breakers. On the other hand, existing mechanical breakers cannot provide sufficient breaking speed. Thus, there is a need for cost- and energy effective fault current handling in an HVDC grid.